An entity is an object that exists and is distinguishable
from other objects.
For instance, John Harris with S.I.N. 890-12-3456 is an entity, as he
can be uniquely identified as one particular person in the universe.

An entity may be concrete (a person or a book, for example) or
abstract (like a holiday or a concept).

An entity set is a set of entities of the same type
(e.g., all persons having an account at a bank).

Entity sets need not be disjoint.
For example, the entity set employee (all employees of a bank) and the
entity set customer (all customers of the bank) may have members in
common.

An entity is represented by a set of attributes.

E.g. name, S.I.N., street, city for ``customer'' entity.

The domain of the attribute is the set of permitted values
(e.g. the telephone number must be seven positive integers).

Formally, an attribute is a function which maps an
entity set into a domain.

Every entity is described by a set of (attribute, data value) pairs.

There is one pair for each attribute of the entity set.

E.g. a particular customer entity is described by the set
{(name, Harris), (S.I.N., 890-123-456), (street, North),
(city, Georgetown)}.

An analogy can be made with the programming language notion of type
definition.

The concept of an entity set corresponds to the programming
language type definition.

A variable of a given type has a particular value at a point in time.

Thus, a programming language variable corresponds to an entity
in the E-R model.

Figure 2-1 shows two entity sets.

We will be dealing with five entity sets in this section:

branch, the set of all branches of a particular bank.
Each branch is described by the attributes branch-name,
branch-city and assets.

customer, the set of all people having an account at the bank.
Attributes are customer-name, S.I.N., street and
customer-city.

employee, with attributes employee-name and
phone-number.

account, the set of all accounts created and maintained in the bank.
Attributes are account-number and balance.

transaction, the set of all account transactions executed
in the bank.
Attributes are transaction-number, date and amount.